The goal of this study Is to address the mechanistic basis for variable rates of protection induced by influenza (flu) vaccination, especially the heterovariant immunity with intranasally administered live attenuated flu vaccines (LAIV) in children and decreased immunity with inactivated flu vaccines (IIV) in the elderly. In Specific Aim 1 we hypothesize that Infection of the upper respiratory tract (URT), the initial site of replication of natural (wild type, wt) flu and LAIV, triggers hierarchical transcriptional signatures in resident epithelial and immune cells that control viral replication and regulate subsequent immunity. This hypothesis will be tested with two approaches. In an ex vivo approach we will infect primary human nasal cells with wt or LAIV viruses and examine: (a) early transcriptional responses and viral replication levels in different epithelial and lymphocytic cell types; (b) the role of specific nasal resident cells in shaping early mucosal immune responses and restricting viral replication; and (c) the relationship between pre-existing adaptive immunity and local immune responses and viral replication. Transcriptional responses will be measured from bulk tissue, purified epithelial and lymphocyte cell subsets, and single infected and non-infected cells using qRT-PCR. These ex vivo results will be extended with an /n vivo approach where we collect flocked nasal swabs from people undergoing wt flu infection or receiving LAIV, measure local transcriptional response and pr e-existing flu specific B/T cells and A b in the blood (in vaccinees) and examine their relationship with the level of viral replication. We will also test the hypothesis that such local antiviral transcriptional responses and viral replication levels predict the subsequent flu specific immune responses. In Specific Aim 2 we hypothesize that wt flu and LAIV induce broader B cell response than IIV in terms of the diversity of clonally expanded Ig gene families and heterovariant Ab reactivity of flu-specific Ig repertoires, especially in young children undergoing the first or second exposure to flu antigens, in comparison to those in the flu-experienced individuals. We predict these early differences affect the evolution of B cell response to new flu exposure later in life. To test this hypothesis we will examine the plasmablast Ig repertoire in infected (wt or LAIV) vs. IIV exposed young children 1 to 4 years of age who are flu-nave (or almost naive) and compare them to their subsequent vaccination and young and elderly adult vaccinees, using a novel barcoding based amplification and sequencing technology as well as production and characterization of recombinant mAbs, with a focus on their homotypic vs. heterovariant reactivity and original antigenic sin that has profound effects on the vaccine efficacy in different age groups.